docs(claude.md): add v1-merge STOP banner directing to migrate-v2.sh

Prepend a Claude-addressed banner so that when an upgrader (or Claude on
their behalf) runs `git pull` / `git merge` from v1 and hits merge
conflicts, Claude aborts the merge and routes the user to
`bash migrate-v2.sh` instead of trying to resolve the rewrite by hand.
Fresh clones are explicitly told to ignore the banner.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -1,3 +1,18 @@
+# ⚠️ STOP — READ THIS FIRST IF YOU ARE CLAUDE ⚠️
+
+**If you are reading this because you just ran `git pull`, `git merge`, `git fetch && git merge`, or any equivalent to bring in upstream changes — and you see merge conflicts or a large diff involving this file — HALT IMMEDIATELY.**
+
+This is NanoClaw v2, a ground-up rewrite with breaking changes throughout. It cannot be merged into an existing v1 install. Attempting to resolve the conflicts by hand, run builds, or "fix" anything will corrupt the user's install and burn tokens for no result.
+
+**Do this instead:**
+1. Abort the merge: `git merge --abort` (or `git reset --hard ORIG_HEAD` if the merge already completed).
+2. Tell the user, verbatim: *"This is the v2 rewrite — it can't be merged into your existing install. Run `bash migrate-v2.sh` to migrate instead."*
+3. Wait for the user to confirm before doing anything else. Do not attempt the migration yourself.
+
+If you are a fresh install (you ran `git clone`, not `git pull`) and there are no conflicts, ignore this banner and continue below.
+
+---
+
 # NanoClaw
 
 Personal Claude assistant. See [README.md](README.md) for philosophy and setup. Architecture lives in `docs/`.